The woodchuck hepatitis virus (WHV) is taxonomically and serologically related to the hepatitis B virus. Infection with each of these viruses is associated with acute and chronic hepatitis and hepatic cell carcinoma in their respective hosts; these associations appear to be etiological in nature. The WHV/woodchuck model system provides a convenient means of studying the relationship between virus and host in the oncogenic process. Tests specific for the WHV antigen-antibody systems have been developed. They are being applied to a prospective study of WHV infection in newborn woodchucks and the ability of active immunization to prevent hepatic cell carcinoma. The results of the study should have important prognostic value in evaluating active immunoprophylaxis of hepatitis B virus in man.